There have conventionally been proposed many similar image retrieval methods using color information and brightness information as image feature amounts. In similar image retrieval using color information (to be referred to as color information retrieval hereinafter), color information is extracted in a registration process for an image subjected to similar image retrieval and image retrieval using an image serving as a retrieval condition. In similar image retrieval using brightness information (brightness information retrieval), brightness information is extracted in a registration process for an image subjected to similar image retrieval and image retrieval using an image serving as a retrieval condition. That is, color information retrieval and brightness information retrieval are realized by independent systems.
Japanese Patent Laid-Open No. 8-249349 discloses an arrangement in which an image is segmented into blocks and pixels in each block are projected into a subspace (color bin) in the color space and the mode color is obtained to calculate an image feature amount.
When an image to be retrieved by color information retrieval is a monochrome or grayscale image, the grayscale region in the color space which expresses the color of the image is merely the region of a line from white to black within the color space. When the color histogram of each block obtained by segmenting an image is generated to extract more accurate color information, blocks assigned to the region of the line from white to black along the grayscale direction are much smaller in number than blocks assigned to another region within the color space. When a monochrome or grayscale image is registered under this condition, the information amount of color information inevitably decreases, and the retrieval precision in retrieval greatly decreases.
When an image to be retrieved by brightness information retrieval is a color image, the grayscale region in the color space which expresses the brightness of the image is merely the region of a line from white to black within the color space, and the brightness information is an 8 quantization(step) expression at most. To the contrary, color information enables a 24-bit expression, and even the same brightness of a color image can be represented by an infinite number of color combinations. It is therefore impossible to accurately retrieve an image whose color coincides with that of a retrieval condition image in retrieving a color image by brightness information retrieval.
As described above, there is no retrieval technique which compensates for the disadvantages of brightness information retrieval and color information retrieval and exploits their advantages.